The present invention relates to a position measuring instrument comprising a scanning device and a measuring graduation, wherein the scanning device is adjustable in directions parallel to and perpendicular to a measuring direction defined by the measuring graduation.
Position measuring instruments may be used to measure the relative position of two objects movable relatively to one another. Typically, the measuring instrument comprises a scanning device, mounted to one of the movable objects, and a measuring graduation capable of being scanned by the scanning device, mounted to the other one of the movable objects. Thus, relative movement of the two objects will produce relative movement of the measuring graduation with respect to the scanning device. Measuring instruments of this type are often used on machine tools to measure the relative position of various machine tool components.
Angle measuring instruments are known wherein the instrument is not supplied by the manufacturer as an assembled device, but rather as components that must be assembled on the customer's machines. If position measurements on these machines are to be precise, then the position measuring instrument must be designed accordingly. This can lead to considerable expense in the assembly and the adjustment of the measuring instrument.
Typically, such measuring instruments comprise a scanning unit and a measuring graduation unit. The graduation unit often comprises a graduation carrier, with an angle graduation in the form of a circle, and is cemented with appropriate accuracy to a centering flange. The graduation unit is then mounted by the customer on a machine shaft and centered for concentricity. The scanning device, with its scanning graduation, frequently is mounted on an assembly surface of the machine and must be very accurately positioned with respect to the angle graduation. This positioning is performed by means of an assembly stop located on the scanning unit. The stop is set against a very precisely manufactured assembly flange on the machine. The assembly flange must be accurately manufactured because any deviation from concentricity with respect to the graduation carrier will produce measurement errors.
Ordinarily, the centering flange of the graduation carrier would be an ideal adjustment aid. However, since the assembly stop of the scanning unit would have to lie against the flange, friction would develop between the centering flange and the assembly stop. Therefore, use of the centering flange as an adjustment aid in this manner is precluded.
This leads to use of the assembly flange of the machine as the adjustment aid. However, since the required tolerances of the assembly flange for positioning purposes are of the order of a few microns, the machine manufacturer may be pushed to the limits of his manufacturing ability.
It is an object of the present invention to create a position measuring instrument that avoids the aforementioned disadvantages, and in which the constructive features required for the exact adjustment are provided by the measuring instrument manufacturer.